Ozone exposure is a growing global health problem, especially in urban areas. While ozone in the stratosphere protects the earth from damaging ultraviolet light, tropospheric or ground level ozone is toxic resulting in damage to the respiratory tract and the exact mechanism by which ozone damages the respiratory system is poorly understood. It has been shown that ozone may be produced endogenously in inflammation and antibacterial responses of the immune system; however, these results have sparked controversy due to the use of a non-specific colorimetric probe.
Reactive oxygen species (ROS) have had a rather controversial history. The existence of singlet oxygen and superoxide was first proposed in the late 1960s; however, due to the lack of sensitive and specific probes these results were met with skepticism, and these observations were only confirmed in the late 1980s. Research indicates that ozone is produced endogenously from singlet oxygen in both neutrophils and atherosclerotic plaque implicating ozone in inflammatory responses. This conclusion was drawn from the premise that the colorimetric dye, indigo carmine 1 (See FIG. 1), differentially reacted with ozone as opposed to other ROS. However, similar to the singlet oxygen/superoxide story, the colorimetric dye, indigo carmine 1 has since been proven to not only react with ozone but also with superoxide in a similar manner, thus, calling into questions these findings of endogenous ozone.
Chemiluminescent methods, including the use of the colorimetric dye, indigo carmine 1, have also been used to measure ozone in ambient air; however, other atmospheric compounds are known to also absorb in this region of the electromagnetic spectrum and false-positive ozone readings are often reported. In addition, many of these methods are sensitive to humidity.
There are several problems associated with the use of the colorimetric dye, indigo carmine 1 as a specific sensor for ozone. First, the dye readily undergoes oxidative cleavage, which is most likely responsible for the lack of specificity. Additionally, the reaction of blue with ROS yields compound 2 (See FIG. 1), which is colorless and cannot be directly visualized without the use of a spectrometer. In biological samples, high-performance liquid chromatography (HPLC) or mass spectrometry are often required in combination with such spectroscopic techniques because the signal for compound 2 (UV: λmax=245 and 298 nm) overlaps with many other bioorganic compounds.
Useful sensors generate virtually no signal in the absence of an analyte, while producing a strong signal in its presence. Small molecule-based fluorescent sensors are particularly useful because they possess spectral properties that are easily discernable and are often soluble in aqueous media. Also, fluorescent samples can be visualized easily with the aid of a hand-held fluorescent spectrometer, fluorescent microscope or even a simple hand-held laser pen.